Atisane and kaurane diterpenes have been isolated from extracts of two trees from tropical rainforests, Homalanthus acuminatus and Chrysobalanus icaco. One compound from each skeletal type was marginally active; three of the isolates are novel compounds. Some insight has been gained regarding structural features important for the bioactivity. The isolates have been used as templates for installing and testing a variety of 2D-NMR experiments.